Field of the Invention
The present invention relates generally to telecommunications systems and methods for connecting calls through a satellite network, and specifically to monitoring delay time from connecting calls over a satellite network.
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, Global System for Mobile Communication (GSM), was established in 1982 to formulate the specifications for mobile cellular radio systems.
With reference now to FIG. 1 of the drawings, there is illustrated a GSM Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a Mobile Station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 12 is divided into a number of cells 22. MS 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS 24 is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several BTSs 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the MSs 20 currently located within the MSC/VLR area 12. If an MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that MS 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
It should be understood that the aforementioned system 10, illustrated in FIG. 1, is a terrestrially-based system. In addition to the terrestrially-based systems, there are a number of satellite systems, which work together with the terrestrially-based systems to provide cellular telecommunications to a wider network of subscribers. This is due to the fact that the high altitude of the satellite makes the satellite visible (from a radio perspective) from a wider area on the earth. The higher the satellite, the larger the area that the satellite can communicate with.
Within a satellite-based network 205, as shown in FIG. 2 of the drawings, a system of satellites 200 in orbit are used to provide communication between MSs 20 and a satellite-adapted Base Station System (SBSS) 220, which is connected to an MSC 14. The MS 20 communicates via one of the satellites 200 using a radio air interface, for instance, based on the Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA). The satellite 200 in turn communicates with one or more SBSSs 220, which consist of equipment for communicating with the satellites 200 and through the satellites 200 to the MSs 20. The antennae and satellite tracking part of the system is the Radio Frequency Terminal (RFT) subsystem 230, which also provides for the connection of the communication path to the satellite 200.
There are currently three different types of satellite systems in place, each having satellites at a different orbit. One type of satellite system is a geostationary (GEO) satellite system, in which the GEO satellites orbit at 36,000 kilometers (km). GEO satellites are often used today for transmission between switches, such as on transatlantic routes. Another type of satellite system is a mid-earth orbit (MEO) system, such as the ICO Global Communications network, in which the MEO satellites orbit at around 10,000 km. The last type of satellite system is a low-earth orbit (LEO) system, such as the Irridium system, in which the LEO satellites orbit at around 100-1,000 km.
During any given call, a number of satellite xe2x80x9chopsxe2x80x9d or connections may be made in order to efficiently complete the call. For example, if a calling MS 20 is located in Asia, while the called MS 20 is located in the United States, there may be several satellite hops involved because the same satellite 200 would not be able to cover both Asia and the United States. In making a decision as to whether to route the call via satellite 200 or through normal trunk lines, a switch, such as an MSC 14, that has received the call typically considers the traffic load, the call type cost for use of the link (satellite or trunk) and the amount of delay that has already accumulated in the call up to that point. The majority of this delay may be due to previous satellite hops. This is due to the fact that each time a call is routed through a satellite 200, there is a resulting propagation delay in the signal. If the accumulated delay is minimal, the MSC 14 may choose to route the call via a satellite 200. However, if the accumulated delay is large, the MSC 14 may decide to route the call through normal trunk lines in order to maintain signal quality.
In order to enable switches to make routing decisions, this delay information is transmitted to the switches in a satellite hop counter field of an Integrated Services Digital Network User Part (ISUP) message. The ISUP message is used to establish connections between switches during call setup. Currently, the satellite hop counter field is incremented based upon the number of GEO satellite hops, in which each hop presupposes a 250 millisecond (ms) delay. Applying the same incrementation method to LEO and MEO satellite hops incorrectly identifies the delay. For example, the Irridium network has the capability to switch traffic between satellites, and thus, several satellite hops may be involved, even though only one satellite hop is recorded.
In addition, a typical MEO delay is approximately 67 ms. It is unclear whether this delay should be counted as a one or as a zero. If it is counted as a zero, and there are four MEO satellite hops in the connection, which is the equivalent of a GEO delay, the counter will not indicate any delay, which is clearly inaccurate. However, if the delay is counted, the counter will indicate a delay of over one second for the four hops, which is also inaccurate. In either case, routing decisions will be made based upon inaccurate delay information.
Furthermore, the satellite hop counter field has been used to generically indicate a delay in the call. This delay can be caused by things other than a satellite hop. For example, Digital Speech Processors within the MSs 20 can cause a delay. Thus, the satellite hop counter may be incremented by one, even if no satellites have been used to make the connection. Therefore, important routing decisions may be made based upon inaccurate and misleading delay information.
It is, therefore, an object of the present invention to provide a more accurate measurement of the cumulative path delay present in a call to enable switches to make an informed decision as to the routing method to use.
The present invention is directed to telecommunications systems and methods for providing a more accurate measurement of the cumulative path delay present in a call to enable switches to make an informed decision as to the routing method to use. In one embodiment, the ISUP satellite hop counter field is expanded to include three fields, one for each type of delay: GEO satellite hops, MEO satellite hops and LEO satellite hops. In an alternative embodiment, a cumulative delay value in milliseconds or centiseconds, instead of the number of satellite hops, can be included in the expanded ISUP satellite hop counter field.